Thanks, Matthias, for highlighting CotEditor which is very nice as a Mac user (I can’t quite recommend it yet as a full TextMate replacement; TM is dead but CotEditor lacks multifile search and replace, absolutely essential for fixing these files).
I would like to highlight that it’s dead simple to copy EUOUAE formulae with the syntax highlighting set up via that scheme (it’s the thing that I copy and paste the most) and it’s pretty easy to set up the highlighting manually, even if you’ve never done it before, which is what I wound up having to do.
Also, although it is a LaTex-specific program not a general text editor and therefore would lack features such as the ability to copy and paste the EUOUAE formulae by clicking it from a special area of the interface or more general syntax highlighting (like the <> or the punctuation needed for bar lines), I got Richard Koch to differentiate braces and brackets and also parentheses in recent updates to TeXShop; since the underlying bits are the same in the program, and with newer LaTeX things relying more and more on both brackets, he graciously made the change for me.
I could surely set up highlighting for TextMate but its development is dead…